1. Field of the Invention
The present invention relates to an exposure apparatus of photolithography equipment used in the manufacturing of semiconductor devices and the like. More particularly, the present invention relates to a light source of the exposure apparatus having a line narrowing module.
2. Description of Related Art
Photolithography equipment is used to form patterns, e.g. circuit patterns, on a substrate in the manufacturing of semiconductor devices and the like. An exposure apparatus of the photolithography equipment generally includes a reticle that defines a pattern to be transferred to a resist on the substrate, a light source for illuminating the reticle with an exposure light such that an image of the reticle pattern is picked up by the exposure light, and a projection lens for focusing the exposure light onto the resist on the substrate. Today's semiconductor devices are being made with increasingly higher integration densities to meet the demand for slimmer and more compact electronic devices. This requires that the exposure apparatus of the photolithography equipment have a light source that emits light having a small wavelength and provides a great depth of focus. Light sources typically employed by exposure apparatus include ultra-high pressure mercury lamps, KrF excimer lasers, ArF excimer lasers, and recently, F2 lasers which are still undergoing testing. An ultra-high pressure mercury lamp emits exposure light having a wavelength of the g-line or i-line of the mercury emission spectrum. A KrF excimer laser emits exposure light having a wavelength (about 248 nm) shorter than that emitted by the mercury lamp. An ArF excimer laser emits exposure light having a wavelength of about 193 nm, and an F2 laser emits exposure light having a wavelength of about 157 nm.
However, the projection lens of a typical exposure apparatus has a different refractive index for different wavelengths of light, and an oscillating laser such as a KrF excimer laser or an ArF laser emits deep ultraviolet light having a relatively large bandwidth. Accordingly, chromatic aberration would occur in an image produced by exposure apparatus which employ a KrF excimer laser or an ArF laser. The chromatic aberration is difficult to correct using additional optics. For this reason, excimer lasers emit monochromatic laser light and the light source also performs what is known as line narrowing to increase the monochromatic characteristic of the exposure light (narrow the bandwidth) and thereby prevent chromatic aberration from occurring in the image. That is, line narrowing allows an oscillating laser, such as a KrF excimer laser or an ArF laser, to be used in an exposure apparatus of photolithographic equipment for manufacturing highly integrated semiconductor devices.
FIG. 1 is a schematic diagram of a conventional laser light source of an exposure apparatus. The light source 10 generally includes a laser oscillator 20, a line narrowing module 30, and an output coupler 40.
The laser oscillator 20 excites a gas mixture, typically including a noble gas such as Ar, Kr, and Ne, and a halogen including fluorine (F), using a laser beam or an electric discharge. The excited F atoms are coupled with gas atoms (e.g., Kr or Ar atoms) in the ground state to create a molecule referred to as an excimer (excited dimer). The excimer exists only in an excited state (not in the ground state), and is unstable. The excimer thus decays within nanoseconds of its formation, thereby discharging ultraviolet light via natural emission and returning to the ground state whereupon its components dissociate. In addition to naturally decaying, excimers are also stimulated to decay in the laser oscillator 20 by UV light that is returned to the laser oscillator 20 by the output coupler 40. In this way, the emission of the UV light is amplified and a portion of the UV light so generated is output as a laser beam. In FIG. 1, reference numeral 21 indicates the region in which excitation occurs in the laser oscillator 20.
Whereas one fraction of the laser beam is output from the laser oscillator 20 to the output coupler 40, another fraction of the laser beam is output from the laser oscillator 20 to the line narrowing module 30 through a rear window 24 of the laser oscillator 20. Inside the module 30, the laser beam passes through a slit 31, is dispersed by a prism beam expander 32, and then is reflected by a total reflection mirror 34 onto a diffraction grating 36. The laser beam is diffracted by the diffraction grating 36 to thereby separate the light which entered the module 30 into different bands or lines. One line, i.e., light having a reduced bandwidth, is returned to the laser oscillator 20. On the other hand, other lines of the laser beam are scattered by a black body in the line narrowing module 30 so as to not return to the laser oscillator 20. Thus, optical energy is wasted.
Meanwhile, the beam input to the laser oscillator 20 is, in turn, input to the output coupler 40 through a front window 23 of the laser oscillator 20. However, only about 20% of that beam is output from the light source due to the provision of a partial reflection mirror 42 of the optical coupler 40. The other 80% is reflected back into the laser oscillator 20 by the partial reflection mirror 42, and stimulates the emission of the UV light in the laser excitation region 21 of the laser oscillator 20. The resulting laser beam is input to the line narrowing module 30 through the rear window 24 of the laser oscillator 20 whereby the above-described processes are repeated.
The depth of focus in a typical process of exposing a layer of resist on a substrate varies by about 0.225 μm per 1 pm of variation in the bandwidth of the exposure light. Thus, the exposure light must have a precise bandwidth if the layer of resist is to be exposed properly enough so that a fine pattern can be formed on the substrate. Preferably, a KrF laser of an exposure apparatus should output exposure light having a wavelength of 248.4 nm and a bandwidth of 0.6 pm when used in a process of fabricating a fine pattern on a substrate. However, in the case of the conventional light source shown in FIG. 1, although the laser beam that is returned to the laser oscillator 20 from the line narrowing module 30 has a reduced bandwidth, e.g., a bandwidth of 0.6 pm, the laser beam comingles with the laser beam created in the laser excitation region 21 of the laser oscillator 20. Therefore, the precision of the reduced bandwidth is considerably degraded. That is, a laser beam having a precise desired bandwidth is not output by the light source.